HBI has attracted attention as a raw material to be charged in a blast furnace which can cope with problems of both the recent tendency to higher tapping ratio operations and reduction of CO2 emission (refer to, for example, Non-patent Document 1).
However, conventional HBI is produced by hot forming of so-called gas-based reduced iron (reduced iron may be abbreviated to “DRI” hereinafter) which is produced by reducing fired pellets with high iron grade, which is used as a raw material, with reducing gas produced by reforming natural gas. Therefore, conventional gas-based HBI is used as a raw material alternative to scraps in electric furnaces, but has a problem in practical use because of its high cost as a raw material for blast furnaces.
On the other hand, there has recently been developed a technique for producing so-called coal-based DRI by reducing, in a high-temperature atmosphere, a low-grade iron raw material with agglomerates incorporated with a carbonaceous material, which contain inexpensive coal as a reductant, and practical application of the technique has been advanced (refer to, for example, Patent Document 1). The coal-based DRI contains large amounts of gangue content (slag content) and sulfur content (refer to Example 2 and Table 7 described below) and is thus unsuitable for being directly charged in an electric arc furnace. In contrast, when the coal-based DRI is used as a raw material to be charged in a blast furnace, large amounts of slag content and sulfur content are not so important problem. In addition, the coal-based DRI has a merit that it can be produced at low cost as compared with conventional HBI.
However, in order to use the coal-based DRI as a raw material to be charged in a blast furnace, DRI is required to have strength enough to resist charging in a blast furnace. The coal-based DRI is produced using a carbonaceous material incorporated as a reductant and thus has high porosity and a high content of residual carbon as compared with gas-based DRI. Therefore, the coal-based DRI has lower strength than that of gas-based DRI (refer to Example 2 and Table 7 described below). Consequently, there is a condition in which in order to directly use the coal-based DRI as a raw material to be charged in a blast furnace, the amount of the carbonaceous material mixed is decreased to extremely decrease the content of residual carbon in DRI (may be abbreviated to “carbon content” (C content) hereinafter), and strength is secured even by the sacrifice of metallization (refer to FIG. 3 of Non-patent Document 2). In addition, like the gas-based DRI, the coal-based DRI is easily re-oxidized and thus does not have weather resistance. Therefore, the coal-based DRI has a problem of being unsuitable for long-term storage and long-distance transport.    Non-Patent Document 1: Y Ujisawa, et al. Iron & Steel, vol. 92 (2006), No. 10, p. 591-600    Non-Patent Document 2: Takeshi Sugiyama et al. “Dust Treatment by FASTMET (R) Process”, Resource Material (Shigen Sozai) 2001 (Sapporo), Sep. 24-25, 2001, 2001 Autumn Joint Meeting of Resource Materials-Related Society (Shigen Sozai Kankeigaku Kyokai)    Patent Document 1: Japanese Unexamined Patent Application Publication No 2001-181721